Cryptococcal meningitis remains a cause of morbidity and mortality in HIV-infected (HIV+) individuals and new modalities are needed for therapy. Profound CD4+ T cell deficiency is found in HIV+ individuals with cryptococcosis, though only a subset of patients develop infection. Therefore, additional immunologic defects probably predispose some patients to cryptococcal infection. In the previous grant period, this group demonstrated differences in the specificity and titer of antibodies to GXM in HIV- and HIV+ individuals. These groups are relatively resistant and susceptible to infection, respectively. The authors showed that human antibodies to GXM are derived from VH3 genes. Together with others' reports of decreased VH3 positive B cells in HIV+ individuals, their findings support the idea that combined with underlying cell-mediated immunity, qualitative and quantitative differences in antibodies to GXM may play a role in the protection of HIV-, and susceptibility of HIV+ individuals to cryptococcosis. This hypothesis will be investigated in this proposal with 3 interrelated aims. First, the authors will use purified antibody subsets from HIV- and HIV+ individuals to determine if idiotype and isotype-specific subsets of antibodies to GXM alter the course of experimental cryptococcosis in normal and immunodeficient mouse strains. In this aim they will also construct and compare VH3 gene libraries from B cells of HIV- and HIV+ individuals with and without cryptococcosis to determine if the frequency of specific VH3 gene segment expression of somatic mutation is correlated with resistance or susceptibility to C. neoformans. Second, they will generate human mAbs to GXM in human immunoglobulin transgenic mice by splenic fusion and determine their VH3 usage, specificity, and functional efficacy in vitro and in experimental cryptococcosis. Third, they will isolate peptide epitopes of the mAbs from phage libraries and use them to compare the specificity of antibodies to GXM from HIV- and HIV+ individuals with and without cryptococcosis. The authors believe that these studies will reveal if antibody repertoire differences contribute to cryptococcal pathogenesis and provide novel human mAbs and peptide mimotopes for passive and active vaccination approaches for therapy of human cryptococcosis.